Magnetic resonance imaging of an osteoid osteoma of the proximal femur: a potentially confusing appearance.

Osteoid osteomas are common benign pediatric bone tumors. However, radiographic diagnosis and location of the tumor nidus can be difficult. We recently diagnosed and treated an osteoid osteoma of the right proximal femur in a 9-year-old girl in whom the preoperative magnetic resonance images demonstrated a well-marginated lesion in the medullary canal but no tumor nidus. This was confusing and misleading. Correct diagnosis was made with a computed tomography (CT) scan. Care must be taken in interpretation of magnetic resonance imaging (MRI) in disorders in which secondary bone marrow changes occur to avoid erroneous diagnoses and possible incorrect operative procedures.